Quick answer
ADU HVAC is the heating, ventilation and air-conditioning system of an accessory dwelling unit. In most small units it is a ductless mini-split heat pump: an outdoor unit linked by refrigerant lines to one or more indoor wall or ceiling units.
The system must be sized to the unit's actual heating and cooling load. In California it must also meet the Title 24 energy code in force when you apply for the permit.
Quick facts
- Most common in small ADUs
- Ductless mini-split heat pump
- Model code sizing rule
- Size equipment to ACCA Manual S or another approved method (2024 IRC M1401.3) 1
- Load calculation
- Manual J works out how much heating and cooling the home needs 2
- California energy code
- 2025 standards apply to permit applications on or after 1 January 2026, and expand heat pump use in new homes 3
- California license
- C-20 warm-air heating, ventilating and air-conditioning 5
- Gas in a new ADU
- Limited in many California cities by all-electric local rules
Key takeaways
- HVAC means heating, cooling and fresh air for the ADU.
- Most small ADUs use a ductless mini-split heat pump. It heats and cools.
- Size comes from a load calculation, not a rule of thumb.
- California's 2025 energy code applies to permits applied for from 1 January 2026.
- Many California cities limit gas in new buildings, which favors heat pumps.
- Refrigerant and electrical work need licensed pros.
What it means, in plain English
HVAC stands for heating, ventilation and air conditioning. It keeps the ADU warm in winter, cool in summer and supplied with fresh air.
A heat pump moves heat instead of making it. In winter it pulls heat from outdoor air into the home. In summer it runs the other way. A mini-split is a heat pump with an outdoor unit and one or more indoor "heads", joined by a small bundle of lines through the wall.
Ductless means no air ducts. Each indoor head heats or cools its own space.
Why it matters to you
Comfort decides whether a tenant stays or a parent is happy living there. A unit that is too hot or too cold is hard to rent.
Your choice also affects the electrical load. A new heat pump may push the house panel toward its limit. See ADU electrical panel for that check.
And it affects permits. In California the system has to meet the energy code. In some cities gas is not allowed in new buildings at all.
HVAC options for an ADU
The table lists the options. The guide under it starts from your floor plan, because closed doors decide more than square feet do.
| System | How it works | Often fits | Trade-offs |
|---|---|---|---|
| Single-zone mini-split | One outdoor unit, one indoor head | Studios and open plans | Closed rooms away from the head may not get enough air |
| Multi-zone mini-split | One outdoor unit, several indoor heads | One- and two-bedroom units | More heads mean more line runs and wall space |
| Ducted mini-split or small ducted heat pump | A compact air handler with short ducts | Units where hidden vents are wanted | Needs space for the air handler and ducts |
| Extend the main house system | Ducts run from the house to the ADU | Some attached units and JADUs | The house system may not have capacity; shared thermostat |
| Gas furnace with AC | Gas heat, separate cooling | Places where gas is allowed | Many California cities limit gas in new buildings |
- If The ADU is a studio or one open room
- Then A single-zone mini-split is often enough, sized by the Manual J load
- If Bedrooms or an office sit behind doors
- Then Plan a head per closed room, or a ducted unit with a supply in each room 2
- If You want no visible wall heads
- Then Look at a ducted mini-split, and find space for the air handler and short ducts
- If The ADU is attached or a JADU and shares walls with the house
- Then Ask for a load check on the house system before you extend it
- If Your city limits gas in new buildings
- Then Plan an electric heat pump and check the panel; see the gas rule in your city
Source: [2]
Why mini-splits are so common in ADUs
Small homes rarely have room for a furnace closet and long ducts. A mini-split needs only a wall spot inside and a pad or bracket outside.
ENERGY STAR describes mini-splits as head units on an interior wall or ceiling with a unit outside and no ductwork. It also notes that heat moves poorly through doorways and cannot pass closed doors. That is why layout matters when you plan heads.
For cold places, ENERGY STAR says its certification for cold-climate air-source heat pumps tests performance down to 5°F.
The diagram shows the parts a ductless system needs. Each one shows up somewhere on your plans or your bid, so you can check nothing is missing.
- Outdoor unitSits on a pad or wall bracket; needs clear space for airflow
- Indoor headsMounted on a wall or ceiling; one per zone 2
- Line setInsulated refrigerant pipes and a cable that join the two units
- Condensate drainCarries away water the indoor head pulls from the air
- Dedicated circuitA breaker and disconnect from the panel; shows on the electrical load calc
- ControlsA remote or wall thermostat for each zone
Source: [2]
Sources: [2]
How an ADU HVAC system gets designed and installed
- A designer or HVAC contractor runs a load calculation (Manual J) for the unit.
- Equipment is chosen to match the load. The model code points to ACCA Manual S for this 1.
- Indoor heads, the outdoor unit and the line route are placed on the plans.
- In California, the energy compliance forms show the system meets Title 24.
- An electrician checks the panel can carry the new load.
- A licensed HVAC contractor installs the system and charges the refrigerant.
- The inspector checks the work before final sign-off.
Example: one head or two?
A hypothetical one-bedroom detached ADU with an open kitchen and living room and a bedroom with a door. This shows the layout logic, not a size or price.
| Space | Door? | Heat path from a head in the living room | Likely plan |
|---|---|---|---|
| Living and kitchen | Open | Direct | Main head here |
| Bedroom | Yes | Blocked when the door is shut | Second head, or a ducted unit |
| Bathroom | Yes | Weak | Exhaust fan plus a small heater if the design calls for it |
The size of each head comes from the Manual J load, not from square feet alone. A bigger unit is not always better: oversized equipment can short-cycle and leave the air clammy.
Where you will see it
- Mechanical plan. Shows heads, the outdoor unit, the line route and fans, in the plan set.
- Energy compliance forms. In California, they show the system meets Title 24.
- Load calculation. A Manual J report, where your city asks for one.
- Electrical load calc. Shows the new circuit the system needs.
- A bid. Equipment, line sets, pad and electrical hookup may be separate lines.
Rules: California first, then other states
California energy code. The California Energy Commission says buildings whose permit applications are made on or after 1 January 2026 must comply with the 2025 Energy Code, which expands heat pump use in new homes.
Which rules apply to your ADU depends on its type and climate zone. See ADU energy code and solar requirements.
California building standards. The 2025 Title 24 code took effect on 1 January 2026.
Local all-electric rules. Some California cities limit gas in new buildings. See all-electric ADU reach codes and the ADU gas connection entry.
Licensing. In California, HVAC work is done under a C-20 license, or a B general contractor running the job.
The model code. The 2024 IRC covers heating and cooling equipment in Chapter 14, including M1403 Heat pump equipment, and requires sizing to ACCA Manual S or another approved method (M1401.3).
Other states. Energy codes vary. See ADU laws by state.
Which energy code applies
In California the date you apply for the permit picks the code. Applications made on or after 1 January 2026 follow the 2025 Energy Code 3. Plans drawn earlier may need updating if you apply after that date. Outside California, your state and city set the edition.
What drives the cost
This page gives cost drivers, not prices.
The drivers are the number of indoor heads, the length of line runs, ducted versus ductless, the efficiency level, any panel upgrade and how hard it is to reach the outdoor spot.
HVAC is a hard cost. Rebates and incentives change often; check the utility and program directly. For overall budgets, see the ADU cost hub.
Ductless mini-split: benefits and limits
Benefits
- Heats and cools in one system
- No ducts to fit in a small unit
- Each head can have its own setting
- Works with all-electric rules
Limits
- Indoor heads are visible on the wall
- Closed rooms may need their own head
- Needs a licensed installer for refrigerant work
- Filters on each head need regular cleaning
ADU HVAC compared with related terms
| Term | What it covers | Read more |
|---|---|---|
| HVAC | Heating, cooling and ventilation | This page |
| Electrical panel | Whether the house service can carry the load | ADU electrical panel |
| Insulation | How well the shell holds heat | ADU insulation |
| Framing | Where heads and lines run through | ADU framing |
Common misreadings
- Myth "Bigger HVAC is safer."
- Reality The model code sizes equipment to a method like Manual S 1. Oversized units can short-cycle.
- Myth "Square feet tell you the size."
- Reality A load calculation looks at windows, insulation, climate and more 2.
- Myth "One head heats the whole unit."
- Reality Heat cannot pass closed doors 2. Bedrooms may need their own head.
- Myth "Heat pumps only cool."
- Reality A heat pump heats and cools by moving heat in either direction.
What to ask your HVAC contractor
- Will you run a Manual J load calculation for this ADU?
- How many heads do we need, and where?
- Does the house panel have room, or is an upgrade needed?
- Where will the outdoor unit sit, and how loud is it near a neighbor?
- Does the system meet the energy code that applies to my permit date?
- Does my city allow gas in a new ADU?
- Are you licensed? (In California, C-20; check CSLB.)
Checklist for your project
- Decide early if gas is allowed or wanted.
- Ask for a load calculation, not a guess.
- Plan head locations with the floor plan.
- Check panel capacity with an electrician.
- Place the outdoor unit away from bedrooms and property lines where possible.
- Confirm energy code forms are in the permit set.
- Keep the manual, warranty and filter schedule for the tenant or occupant.
Upkeep for owners and tenants
- Clean or replace the filters in each indoor head on the maker's schedule.
- Keep the outdoor unit clear of leaves, plants and stored items.
- Watch for water dripping from an indoor head. It can point to a blocked drain line.
- Book a licensed technician for refrigerant or electrical checks.
- Put the manual and remote instructions where the occupant can find them.
Questions about adu hvac
What is the best HVAC system for an ADU?
For most small ADUs, a ductless mini-split heat pump. It heats and cools, needs no ducts and fits all-electric rules. Layout, climate and budget can point to a ducted system instead.
What size mini-split do I need for an ADU?
It depends on a load calculation, not square feet alone. Manual J works out the load, and equipment is matched to it. The 2024 IRC points to ACCA Manual S for sizing.
Can an ADU share HVAC with the main house?
Sometimes, mainly for attached units or junior ADUs. The house system must have spare capacity, and both homes share control. Most detached ADUs get their own system.
Does an ADU need air conditioning?
Not always by code, but heating is usually required and cooling matters in hot climates. A heat pump does both, which is one reason it is common.
Are heat pumps required for ADUs in California?
It depends on the ADU type, climate zone and permit date. The 2025 Energy Code, for permits applied for from 1 January 2026, expands heat pump use in new homes. Ask your building department.
Can I use gas heat in a new ADU in California?
Maybe. Some cities limit gas in new buildings. Check your local rules; the all-electric reach codes research page tracks them.
How many indoor heads does an ADU need?
Often one for a studio and two or more where bedrooms have doors. Heat cannot pass closed doors, so rooms that close off often need their own head.
Will a mini-split overload my electrical panel?
It adds load. An electrician runs a load calculation to see if the panel can carry it, or if an upgrade is needed.
Who can install ADU HVAC in California?
A contractor with a C-20 HVAC license, or a B general contractor running the job. Refrigerant handling also needs certified technicians. Check the license on CSLB.
Can I install a mini-split myself?
This page does not give DIY steps. Refrigerant, electrical and permit work are for licensed pros. A bad install can void the warranty.
How do I maintain an ADU mini-split?
Clean each head's filter on the maker's schedule, keep the outdoor unit clear and watch for drips. A licensed technician handles refrigerant and electrical checks.
Where should the outdoor unit of an ADU mini-split go?
On a level pad or wall bracket with clear airflow, as the maker specifies. Keep it away from bedroom windows and property lines where you can, and check any local setback or noise rules with your city.
Sources
- International Code Council, 2024 International Residential Code, Chapter 14 Heating and Cooling Equipment and Appliances, retrieved .
- ENERGY STAR (US EPA), Ductless Heating and Cooling, retrieved .
- California Energy Commission, 2025 Building Energy Efficiency Standards, retrieved .
- California Building Standards Commission, California Building Standards Code (Title 24), retrieved .
- Contractors State License Board, License Classifications, retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: 2024 IRC Chapter 14 (M1401.3 text, M1403 heading) was read on codes.iccsafe.org; ENERGY STAR, CEC, CBSC and CSLB pages on 8 October 2026. No prices or equipment sizes are given.
Suggest a correction. We fix errors and say what changed.